Circuit design for the latest electronic equipment, including TVs, radios, computers, medical devices, office equipment and communications devices, has become increasingly complex. For example, integrated circuits which contain the equivalent of several hundreds of thousands of transistors are now manufactured for these and other kinds of equipment. This rise in design complexity has been accompanied by a parallel trend toward the fabrication of ever smaller electronic components. That is, manufacturers are finding ways to fit larger numbers of such components on steadily shrinking device footprints while at the same time continuing to reduce the dimensions of the device.
These electronic components—particularly central processing units (CPUs), drivers, integrated circuits (ICs), memories and other large-scale integration (LSI) devices—which are used in electronic equipment such as personal computers, digital video disks and cell phones generate more heat as a result of the increasing level of integration. Such heat generation raises problems of failure or inoperability. It is thus needed to effectively dissipate the heat generated by electronic components.
Many heat dissipating methods, as well as heat dissipating articles and compositions used in such methods, have already been devised to reduce the heat generated by electronic components. Heat sinks in the form of plates made of brass and other high thermal conductivity metals are used in electronic equipment to hold down the rise in temperature of electronic components therein during use. These heat sinks carry away heat generated by the electronic components and release that heat from surfaces by means of a temperature difference with outside air.
For heat generated by an electronic component to be efficiently transferred to a heat sink, it is necessary that the heat sink be placed in close contact with the electronic component. Because of height differences among various electronic components and component tolerances in the assembly process, a flexible heat conductive sheet or a heat conductive grease is often placed between the electronic components and the heat sink so that heat transfer from the electronic components to the heat sink takes place through the heat conductive sheet or grease. Heat conductive sheets made of heat conductive silicone rubber or the like (heat conductive silicone rubber sheets) are used as the heat conductive sheet.
However, the heat conductive grease is awkward to handle (or to process), and problems with the heat conductive sheets are their interfacial thermal resistance and insufficient actual heat dissipation.
Then JP-A 2000-509209 proposes a phase transition heat dissipating sheet, which is poor in flame retardance, heat resistance and weather resistance. JP-A 2000-327917 proposes a silicone base thermosoftening composition, which has poor flame retardance and heat resistance due to the special silicone base wax used, and lacks flexibility when formed into a sheet.